"The failure of (the Homeland Security Department) or OPM systems to detect the breach does not indicate a level of sophistication on behalf of the adversary; rather, it only shows that the breach was sophisticated for 1970s legacy systems that operate on COBOL mainframe applications that have not been updated since the Y2K bug," the brief stated.

The think tank recommended the replacement of antiquated cyber defense systems, such as firewalls and antivirus programs, and the adoption of more innovative programs that can adapt and respond to specific situations.

Advanced, unrelenting threats are often designed to modify sophisticated intrusions, catering them to specific victims or organizations.

"Novel malware can bypass detection, avoid run-time analysis and prevent post-incident traces in a number of ways undetectable to current defense-in-depth norms," the report stated. "It is as effective as trying to stop a laser pointer with a chain link fence."

The brief also recommended the implementation of a behavioral analytics system, which creates a baseline profile of a user and detects and reports abnormal activities.

"Training remains the easiest and best strategy to mitigate adverse effects of the OPM breach such as insider threats, spear phishing emails, social engineering or future breaches," the report stated.

The U.S. Office of the Inspector General also issued a report, "Postal Service Cybersecurity Functions," on a November 2014 cyber-intrusion.

In November, the USPS announced a cyberattack compromised the personal information of more than 800,000 current and former employees, compensation records from 485,000 individuals and customer inquiries from about 2.9 million customers, according to the report.

According to the Inspector General, the USPS had under-trained employees, outdated technology and ineffective collaboration among cybersecurity teams.

The USPS was also using outdated systems no longer supported by the vendor, and therefore did not have security patches for vulnerabilities. The outdated systems included just 16 of 31 software versions and nine operating systems across 39 servers, the report said.

Since the 2014 breach, the USPS has been elevating cybersecurity, the OIG acknowledged. But its recommendations to the Postal Service included providing more funding for "proactive prevention, detection, response and mitigation of sophisticated cyber threats."

In a response included in the report, USPS' acting chief information officer and executive vice president Randy Miskanic said the agency generally agrees with the broad intent of most of the recommendations in the report, but believes the cyber-threats it faces warrant more flexible and active management processes and modes of response than those identified by the OIG.